Polyaniline has emerged as one of the most promising conducting polymers, because of its excellent chemical stability combined with respectable levels of electrical conductivity of the doped or protonated material. Polyaniline is also of special interest because the electrical properties of polyaniline can be reversibly controlled both by oxidation and by protonation. Thus, a wide range of associated electrical, electrochemical, and optical properties, coupled with excellent environmental stability, make polyaniline a useful material in a wide variety of technologically important applications.
Investigation has been conducted in recent years with a view towards applying polyaniline materials in a wide variety of applications such as electrode materials, batteries, antistatic materials e.g., paints, adhesives, films, electromagnetic shielding materials, functional devices-e.g., photoelectric transducers, optical memories and various sensors, display devices, various hybrid materials, transparent electroconductors, LED, electro-optic devices photocells, circuit boards, etc. electron beam photoresistive agents, and various terminal equipment.
Polyaniline can be synthesized from monomer by both electrochemical and chemical oxidative polymerization methods. Electrochemical oxidation utilizes an electrochemical charge transfer reaction, whereas chemical oxidation occurs by means of chemical reaction with an appropriate oxidizing agent.
The chemical oxidative polymerization of aniline is particularly important since this mode of synthesis is the most feasible for large-scale production of polyaniline. The chemical oxidative polymerization of aniline routinely is carried out in acidic aqueous solutions.
Synthesis of polyaniline salt (emeraldine salt) is commonly performed by the method of chemical oxidative polymerization based upon an aqueous solution polymerization system. (see Cao et al., Polymer, 30:2305, 1989; Palaniappan, Polymers for Advanced Technologies, 5:295, 1994). This method involves combining water, a protonic acid, aniline and an oxidizing agent and allowing the mixture to react while maintaining the reaction mixture at constant temperature. After a period of several hours, the precipitated polyaniline is separated from the reaction mixture by filtration and washed. The polyaniline obtained in such way is normally insoluble, which hinders the application of the polyaniline.
Harlev et al., U.S. Pat. No. 5,618,469, disclosed a method for preparing a processable solution of emeraldine salt for use in the preparation of electrically conductive and optically transparent coatings, and to articles, such as liquid crystal devices and other electro-optic devices. Such method comprises oxidatively polymerizing aniline monomer in the presence of a protonic acid, for example pyruvic acid, to yield a solid emeraldine salt, reacting the solid emeraldine salt with an aqueous ammonia solution to form a solid emeraldine base, and dissolving the emeraldine base in an additional amount of pyruvic acid to form a solution of polyaniline. Abe et al., U.S. Pat. No. 5,728,321, disclosed a solution of polyaniline (dissolved in an aprotic polar solvent, such as 1-methyl-2-pyrrolidone) in doped state can be obtained by a method using a specific protonic acid, such as hydrofluoroboric acid, hydrofluorophosporic acid, perchloric acid, or any other organic acids having acid dissociation constant pKa values of less than 4.8, as dopants in the oxidative polymerization of aniline monomer. Some disadvantages of these methods are that the emeraldine salt formed in the reaction mixture during the oxidative polymerization is insoluble and precipitated out so that a complex process is needed to transform the salt into a solid emeraldine base which is then dissolved in acid to form into a processable solution of polyaniline. The undoping of doped polyaniline in order to permit the polyaniline to be soluble in organic solvent is burdensome and increases the production cost.
Routes towards soluble polyaniline include the preparation of polyaniline derivatives and copolymers. (Mav et. al. Journal of Polymer Science, Part-A, Polymer Chemistry, 38: 3390, 2000; Norris et. al. Macromolecules, 33: 3237, 2000; Xin-Gui Li et. al. Journal of Polymer Science, Part-A, Polymer Chemistry, 38: 4407, 2000). Unfortunately, these species invariably show significantly reduced conductivities in comparison with the (unmodified) homopolymer.
Smith et al., U.S. Pat. No. 5,470,505, disclosed that the emeraldine salt prepared by standard methods of oxidative polymerization of aniline monomer in the presence of a protonic acid can be dissolved in an acid, particularly strong acid such as concentrated H2SO4, CH3SO3H, ClSO3H, CF3SO3H and HNO3 (70% or fuming). The emeraldine salt (polyaniline) dissolved in one of these acid solutions is then processed into desired articles in the applications.
To improve the processability, emulsion polymerization processes for preparing a polyaniline salt of a protonic acid such as dodecylbenzene sulfonic acid, camphor sulfonic acid have been reported. (Cao et al., U.S. Pat. No. 5,232,631, Example 6B, 1993; Smith et al (Polymer 35, 2902, 1994; Terhi Vikki et al., Macromolecules, 29, 2945, 1996; Kinlen et al., Macromolecules, 31, 1735, 1998; and Olinga et al., Macromolecules, 33, 2107, 2000). In these disclosures aniline, a functionalized protonic acid, and an oxidant were combined with a mixture of polar liquid, typically water and a non-polar or weakly polar liquid, e.g. xylene, chloroform, toluene, resorcinol, decahydronaphthalene and 1,2,4-trichlorobenzene, all of which are not completely soluble.
Applicants have earlier filed patent applications for a process for preparation of polyaniline salts in organic solvents is reported. (Pending Indian Patent Application NO.74/DEL/2002 and U.S. patent application Ser. No. 10/098,188 filed on Mar. 13, 2002). The polyaniline salt is in a carrier solvent such as chloroform, dichloromethane, benzene, toluene, xylene and the solution is optically transparent.
Polyaniline salt has been categorized as an inter-actable material, which is neither soluble nor fusible under normal conditions. Several strategies were worked out to introduce solubility and processability in polyaniline. They are                Dedoping of polyaniline salt to polyaniline base. Dissolving polyaniline base in aprotic solvent and redoping to polyaniline salt. This procedure is burdensome and increases the production cost.        Preparation of substituted polyaniline; preparation of polyaniline copolymers, which are not, the homopolymer of polyaniline salts. The conductivity of the substituted polyaniline and copolymer may be much lower than that of the polyaniline.        Dissolving the polyaniline salt in concentrated acid. However, they are highly corrosive because the use of concentrated acid.        Preparation of polyaniline salt using functionalized protonic acids by emulsion polymerization process.        
In the present invention, a process for preparation of polyaniline salt is reported. The polyaniline salt prepared is in carrier solvents and soluble in solvents and mixture of solvents. Polyaniline salt is also prepared in powder form, which is soluble in solvents and mixture of solvents.